Power devices are commonly used in many applications wherein it is necessary to provide high currents (e.g., up to 1-100 A), so as to be able to drive high inductive loads (e.g., for controlling motors in industrial applications) or capacitive loads, and withstand high voltages at their terminals (e.g., up to 400-1,600V).
For example, a widely used class of power devices is represented by IGBTs (“Insulated Gate Bipolar Transistor”), which are able to combine advantages being offered by field effect transistors (MOSFET, or simply MOS) and bipolar transistors (BJT), i.e., voltage driving and low output resistance, respectively.
An IGBT device is formed by a MOS transistor (e.g., an N-channel MOS transistor) and a bipolar transistor (e.g., a PNP bipolar transistor) coupled to each other; in particular, a base of the bipolar transistor may be coupled to a drain of the MOS transistor, whereas a collector of the bipolar transistor may be coupled to a source and a body of the MOS transistor. The IGBT device has an emitter terminal coupled to the source of the MOS transistor, a collector terminal coupled to an emitter of the bipolar transistor, and a gate terminal coupled to a gate of the MOS transistor; when a command signal is applied to the gate terminal, the IGBT device can enable a corresponding current to flow between the emitter terminal and the collector terminal thereof.
In this way, during the operation of the IGBT device, the bipolar transistor has a conductivity modulation effect on the drain of the MOS transistor; this implies a reduced voltage drop across the MOS transistor terminals and a high available current density, which in turn implies the possibility of integrating a very large number of IGBT devices in a same chip of semiconductor material.
Although IGBT devices have a widespread diffusion and are largely used, they have some drawbacks that preclude a wider use thereof, for example, in particular applications that require high performance and reliability.
In fact, as it is known, each IGBT device typically has a composite structure including layers with different types and/or concentrations of doping being alternated to each other; such a structure usually introduces undesired parasitic elements that may modify the correct operation of the IGBT device, or even cause the breakdown thereof even in a relatively short time.
Such parasitic elements mainly include a parasitic resistor coupled between the collector of the bipolar transistor and the emitter terminal of the IGBT device, and a parasitic bipolar transistor (with polarity being opposite that of the bipolar transistor—i.e., of NPN type in the case at issue); the parasitic transistor gives rise, in combination with the bipolar transistor, to a parasitic thyristor that may be enabled by an excessive potential difference across the parasitic resistor.
In particular, during the operation of the IGBT device, a certain possibility exists that leakage currents present therein pass through the parasitic resistor, thereby causing a potential difference across it that might reach such a level to turn on the parasitic transistor. Moreover, if during such condition a variation of electrical parameters of the bipolar transistor and the parasitic transistor (e.g., current gain) also occurs, then the parasitic thyristor may cause an uncontrolled current path between the emitter terminal and the collector terminal of the IGBT device, thereby triggering a self-generating effect of current multiplication (known as latch-up) that typically causes the breakdown of the IGBT device.